1. Field of Invention:
This invention relates generally to work transfer systems, and more particularly to an automated system in which a robot running on a rail along the front of a work bench having an assembly of processing tanks at various positions thereon, acts to convey a work basket in a programmed sequence to each of the tanks, the robot acting to properly position and orient the work basket with respect to each tank before the basket is immersed therein.
2. Prior Art:
By using diffusion techniques it is now possible to fabricate transistors and diodes as well as resistors and capacitors within a single wafer of silicon to create integrated circuits. The manufacture of microelectronic devices entails sequences of photolithograaphy, etching and critical cleaning processes. For the most part, these have heretofore been carried out manually at wet chemistry work benches.
In the usual manufacturing procedure, batches of substrates or silicon wafers are carried in "boats" and transferred from one chemical bath or tank to another by a human operator who adjusts the conditions prevailing in each tank and also determines the periods during which the boat dwells in the respective tanks for treatment therein. Typically, the work bench arrangement is such that one or sometimes two rows of chemical and rinsing tanks are provided and in which the various processing baths are not usually in accurate alignment with respect to each other and in fact may be randomly placed.
The chemical and rinsing baths are generally accommodated in a console which also incorporates a clean air environment. To avoid contamination of the work being processed, the basket holding the workpieces are manually transferred by the operator from one chemical or rinsing bath to another from left to right on the work bench, or fore and aft depending on the placement of the tanks, in a timed sequence dictated by the processing procedures appropriate to the work pieces being handled.
To carry out this timed sequence, the operator must from time to time reach into the clean environment and across various processing baths in response to bells and buzzers timing the process. This manual procedure gives rise to a margin of human error in the timing of any one process or in the processing sequence, and it also may result in contamination of the work pieces. The need exists, therefore, for an automated system to transfer work pieces from one chemical bath to another by precisely controlled mechanisms which dispense with human operators.
While automated transfer operations are now commonplace in many industrial processes including electroplating, a number of practical factors have heretofore militated against the introduction of automation in the processing of microelectronic devices. A major factor is that the value of the microelectronic devices is high relative to the cost of labor involved in making these parts. Thus the introduction of automated work transfer mechanisms to reduce labor costs may not be economically justified particularly when one takes into account the capital investment dictated by automated equipment.
But other factors now come into play which have little to do with the cost effectiveness of replacing manual operations with automated work transfer mechanisms. As microelectronic devices become increasingly complex and sophisticated, the associated wet chemistry procedures are rendered even more critical. As a consequence, the variables and human errors incidental to manual operation can no longer be tolerated, for repeatability and consistency are now the primary desiderata. Hence, these considerations may override the cost effectiveness factor.
Though efforts have been made to automate work transfer operations in wet chemistry processing of microelectronic devices, they have had limited success, largely because of certain environmental problems. Many of the chemical processing tanks used in micro device and wafer processing act to discharge corrosive vapors into the work area in the region immediately above the process tanks. Since the hoist is for the most part required to operate within this corrosive environment, freely moving, unanchored hoist type of work transfer mechanisms such as are employed in the printed circuit boart plating industry have not been readily adaptable to this quite different and specialized type of use.
Instead, use has been made of fixed-sequence transfer mechanisms. These enjoy the advantage of lesser vulnerability to the corrosive environment by reason of the reduced complexity of these parts of the mechanism which are exposed to the environment. However, they suffer from the limitation that work transfer takes place only between two immediately adjacent tanks. In general, a single, commcn processing cycle time must be used.
As a consequence, the corrosive nature of the environment associated with many of the wet chemistry processes in microelectronic device and wafer fabrication has hitherto tended to discourage the use of an unanchored, horizontally-free tracking hoist or robot in this industry.
The Layton U.S. Pat. No. 466,454 (1984) discloses a work transfer system in which a robot is provided having a hand adapted to grasp the handle of the work basket and to manipulate the basket in accordance with programmed instructions, whereby the basket may be made to carry work in any desired sequence along a row of processing tanks disposed on a work bench. In this system, the entire transfer mechanism, but for the robot hand and its supporting arms, lies outside of the corrosive environment and need not, therefore, be protected against corrosion.
In this prior system, a rail is disposed adjacent the front of the work bench in parallel relation to the uniform row of tanks thereon. Riding the rail is a robot having a pair of vertical shafts extending upwardly therefrom, the shafts being supported on an elevator platform disposed within this robot. A pair of arms cantilevered from the upper ends of these shafts extends over the work bench, the extremities of these arms being pivotally connected to a cross piece from whose center depends a rod terminating in a hand adapted to engage the work basket handle.
In operation, when the robot is instructed to carry the work basket to a selected tank, the robot is caused to travel on the rail to a position at which the hand and the basket borne thereby are in general alignment with this tank. Concurrently, the shafts are rotated to angle the cross piece and thereby turn the rod carrying the hand to orient the work basket so that it is properly oriented with respect to the selected tank therebelow. And by then operating the robot elevator, the work basket is lowwered into the tank for processing, the basket being thereafter lifted from the tank so that the basket can then be transferred to another processing tank on the work bench.
There are, however, some work bench tank set-ups for which the prior system disclosed in the Layton patent is unsuited. Thus, where instead of a uniform orientation of tanks as shown in this patent, there may be more than one orientation of such tanks on the bench. In a situation in which the tanks are randomly dispersed or are not oriented in the same direction, the complex motions then required of the work transfer system to properly position and orient the basket with respect to a given tank cannot be executed by the prior system disclosed in the Layton patent.